Temperature sensors have numerous applications such as for determining the temperature of gases, for example, in automobile engine applications. Traditionally, most sensors have taken the form of sensing elements such as thermostats having switched outputs. However, with the advent of complex electronic engine controllers, it is desirable to provide an absolute indication of the sensed temperature rather than merely a switched output.
Hence, modern sensors typically incorporate passive devices such as thermistors, negative thermal coefficient devices (NTC) or positive thermal coefficient devices (PTC). The output of these passive devices is typically in the form of a resistance which varies with temperature. Extreme conditions such as water, thermal cycle, and vibration can lead to a degradation of the wiring harness used to connect the sensor to the engine controller. For example, moisture can induce conductivity between individual wires in the wiring harness and over time the resistance of the individual wires can change. Such wiring harness degradation can induce inaccuracies in the signal received by the engine controller.
Past sensors suffer from further disadvantages because they are typically constructed with metal housings. Known devices involve fitting the sensing element into a housing constructed of brass or other similar metals. The housing typically includes an exterior thread and a hexagonal portion enabling the device to be screwed into a reciprocal threaded aperture in an engine block, for example. One significant disadvantage of metal housings, and particularly those formed with a hexagonal nut portion, is that the metal acts as a heat sink and draws the heat away from the sensing element, thereby causing inaccurate temperature readings. Another significant disadvantage is that when heat is applied to the backend of the sensor, energy is conducted to the temperature sensing element, causing even greater inaccuracies.
Typically, the temperature sensing element is coated with a layer of epoxy and then exposed to the gas medium. The epoxy is intended to protect the element from damage from, for example, moisture, while not affecting the response of the element. However, testing has shown that moisture "follows" the surface of the wires connected to the element. Damage due to the moisture inevitably follows.
Temperature sensors designed to sense the temperature of gases are especially intolerant of external heat. Heat in other applications where the sensor is surrounded by liquid is transferred to the surrounding liquid. Thus, a sensor adapted for use in a gaseous environment must be able to handle without losing accuracy the additional energy that is not transferred away from or towards the sensor. In other words, a gas temp sensor must be more responsive and require less energy expense to heat the element.
The present invention is directed towards one or more of the problems discussed above.